The present invention relates in general to detecting and reducing adjacent channel interference in a radio receiver, and more specifically to determining the presence of an upper adjacent channel or a lower adjacent channel and shifting the frequency of a mixing signal to reduce adjacent channel interference.
It is well known that the commercial AM and FM broadcast bands include a plurality of evenly spaced channels. A particular broadcast station is allocated a channel for broadcast within an assigned frequency range.
The power spectrum of a transmission depends on the energy content of a radiated signal at each frequency. Even though most energy in a transmission can be limited to its assigned channel, efficient use of all the channels implies that some radiated energy will be at frequencies outside the assigned channel.
Assignment of broadcast channels to transmitters is determined according to geographic location and other factors to minimize interference between tranmissions in adjacent channels. However, demand in populous areas is high for the limited number of channels. A radio receiver must therefore cope with the situation where a strong signal on an adjacent channel creates signal components in the desired channel which interfere with reception of the desired signal. Interference has been found to be objectionable when the total power in the adjacent channel signal is about 30dB greater than the total power in the desired channel signal.
Some prior art radio receivers have detected the presence of an objectionable adjacent channel signal by various methods including: separately tuning each channel and measuring its signal strength; detecting beat components caused by an adjacent channel in the desired channel; or detecting the difference in signal levels of a narrowband portion of the desired signal and the full wideband signal in the desired channel. If no adjacent channel signals are found in the prior art receivers, then a wideband intermediate frequency (IF) filter is used to give the maximum signal quality to the desired signal. If an adjacent channel signal is found, then a narrowband IF filter is switched into the signal path to eliminate adjacent channel interference at the expense of introducing modulation distortion which reduces the desired signal quality. The prior art also teaches a receiver wherein narrowband and wideband IF signals are blended together in accordance with the relative strength of the adjacent channel signals so that switching between the narrowband and wideband IF signals is not noticeable. However, modulation distortion is still introduced.